Abstract: The paper investigates the mechanism of additional hardening of materials under multi-axial non-proportional loading, and then studies the state of the strain on the critical plane of thin-walled tubular specimens. The plastic strain energy for control parameter is used to define the non-proportionality to characterize the influence of strain path on the multi-axial low cyclic fatigue life. The model for predicting multi-axial low cycle fatigue life is proposed by means of von-Mises criterion and the non-proportionality state of strain path based on the critical damage plane of multi-axial fatigue, which can determine the effect of strain path on the non-proportional additional hardening. Multi-axial fatigue life of 08X18H10T stainless steel, Ti-6Al-4V alloy, S460N steel and 2.25Cr-1Mo steel are predicted by this model, and calculated results are compared with experimental data. The results indicate that predicted results are in good agreement with experimental results, and the model can be applied to both proportional and non-proportional loading conditions. It is convenient for engineering application with high precision.